A position light signal is one where the position of the lights, rather than their color, determines the meaning of the aspect that's being conveyed to the Engineer or other designated employee leading the move. The aspect consists solely of a pattern of illuminated lights, which are all of the same color (typically amber or white). In many countries, small position light signals are used as shunting signals, while the main signals are of color light form.

On the Long Island Rail Road as on other railroads, initial efforts were made to replace the semaphore signal's with the illumination of the position of the blade rather than by color lamps alone. This had the advantage of eliminating any and all effects of even slight color blindness by the train crew. (The PRR realized that 1 in 3 engineers were color blind). Lamps with inverted half toric optic lenses, covered with a light yellow tinted conical cover glass with a frosted tip to avoid phantom indications were displayed in rows of three, corresponding to the positions of a semaphore blade. Multiple signal heads were used at interlockings where four aspects did not suffice. The Pennsylvania Railroad chose to use their Superintendent of Signaling, A.H.Rudd's, in-house developed position light signals to both replace the semaphores and their moving parts, also because the intense lemon yellow light provided superior visibility in adverse weather conditions such as rain, fog or snow. The prototype position lights used rows of four lamps in an asymmetric fashion in the style of semaphore blades, but this was later changed to the symmetric three-lamp system. The first installation of the four 5 volt 10 watt lamp position light signals occurred on the Mainline between Philadelphia and Paoli, in conjunction with the 1915 electrification. These first signals differed from the later ones in that the lamps were mounted separately in front of a tombstone-shaped black painted metal backing. There were found issues with wind damage due to the rather larger "sail area" of the "tombstone backing." Soon thereafter, the lamps were reduced in number to three per row, without adversely affecting the long range viewing of the signal aspects and the backing correspondingly reduced in size and made as a disc. The lamp units and background disc on a mounting system known as a "spider," were integrated into a single unit.
Below, you will see a chart of position light signals, with an accompanying rule number. 
This is an over view of the LIRR/PRR system.
We will take each individual Position Aspect and describe what it means.

Note: On the LIRR the "Dwarf" signals are referred to as "Low-Homes", These are only found in interlockings.  

The "Dwarf Pedestal" is referred to simply as "Pedestal(s)"

In simplest terms, a "home signal" is an interlocking signal or

non-interlocked signal which displays "STOP SIGNAL" (Rule 292) as its most restrictive aspect